Forming three-dimensional objects by photochemical hardening of successive contiguous layers on a workpiece support surface within a liquid resin bath disposed in a vessel has been known for more than 40 years. The commercialization of this relatively old concept has produced numerous patents as computer technology has developed to maintain the precision required to make the stereolithograghic process efficient enough to effect commercial production of three-dimensional articles.
From the outset as early as 1956, the need for precision control of the amount of photohardenable liquid being disposed over the next previous layer of hardened polymer material was recognized. The industry thus developed equipment and methods for controlling the known parameters for producing each successive layer in the required series to form the three-dimensional object. The known parameters for the liquid medium layer formation process include maintaining the liquid resin level within the resin vat and controlling the application and layer thickness of the liquid layer medium to be selectively hardened to form the desired cross-sectional slice of the article being formed.
Many systems for production of three-dimensional modeling by photoforming are known. European Patent Application No. 250,121, filed by Scitex Corporation Ltd., on Jun. 6, 1987, discloses a three-dimensional modeling apparatus using a solidifiable liquid, and provides a good summary of documents pertinent to this art. U.S. Pat. No. 4,575,330, issued to C. W. Hull on Mar. 11, 1986, describes a system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction. Successive adjacent laminae or layers, representing corresponding successive adjacent cross-sections of the object, are automatically formed and integrated together to provide a step-wise laminar buildup of the desired object whereby a three-dimensional object is formed end drawn from a substantially planar surface of the fluid medium during the forming process.
U.S. Pat. No. 4,752,498, issued to E. V. Fudim on Jun. 21, 1988, describes an improved method of forming three-dimensional objects, which comprises irradiating an uncured photopolymer by transmitting an effective amount of photopolymer solidifying radiation through a radiation transmitting material which is in contact with the uncured liquid photopolymer. The transmitting material is a material which leaves the irradiated surface capable of further crosslinking so that when a subsequent layer is formed it will adhere thereto. Using this method, multilayer objects can be made.
A publication entitled "Solid Object Generation" by Alan J. Herbert, Journal of Applied Photographic Engineering, 8(4), 185-188, August 1982, describes an apparatus which can produce a replica of a solid or three-dimensional object much as a photocopier is capable of performing the same task for a two-dimensional object. The apparatus is capable of generating, in photopolymer, a simple three-dimensional object from information stored in computer memory. A good review of the different methods is also given by a more recent publication entitled "A Review of 3D Solid Object Generation" by A. J. Herbert, Journal of Imaging Technology 15:186-190 (1989).
Most of these approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are describe as well as the use of direct laser writing, that is, exposing a photoformable composition with a laser beam according to a desired pattern and building a three-dimensional model, layer by layer. In addition to various exposure techniques, several methods of creating thin liquid layers are described which allow both coating a platform initially and coating successive layers previously exposed and solidified.
The foregoing prior art methods of coating are not capable of ensuring flat uniform layer thickness or of producing such layers quickly, however. They do not effectively prevent damage or distortion to previously formed layers during the successive coating process and they involve coating only liquid formulations of preferably low viscosity. Furthermore, they do not recognize very important parameters involved in the coating process such as the effects of having both solid and liquid regions present during the formulation of the thin liquid layers, the effects of fluid flow and rheological characteristics of the liquid, the tendency for thin photoformed layers to easily become distorted by fluid flow during coating, and the effects of weak forces such as hydrogen bonds and substantially stronger forces such as mechanical bonds and vacuum or pressure differential forces on those thin layers and on the part being formed.
The Hull patent, for example, describes an over-dipping process where a platform is dipped below the distance of one layer in a vat, then brought up to within one layer thickness of the surface of the photoformable liquid. Hull further suggests that low viscosity liquids are preferable but, for other practical reason, the photoformable liquids are generally high viscosity liquids. Motion of the platform and parts, which have cantilevered or beam regions (unsupported in the Z direction by previous layer sections) within the liquid, creates deflections in the layers, contributing to a lack of tolerance in the finished part. In addition, this method is rather slow.
U.S. Pat. No. 2,775,758, issued to O. J. Munz on Dec. 25, 1956 and the Scitex application describe methods by which the photoformable liquid is introduced into a vat by means of a pump or similar apparatus such that the new liquid level surface forms in one layer thickness over the previously exposed layers. Such methods have the aforementioned disadvantages of the Hull method except that the deflection of the layers during coating is reduced.
Munz calls his stereolithographic system, a photo-glyph recording system. The patent discloses a liquid photographic formulation that is self-developing, self-fixing and solidifying because it is photo-hardenable on exposure to light. Any conventional photo-sensitive material such as the photopolymer disclosed in the 1945 British Patent 566,975 may be used in the Munz patented process and apparatus.
Munz uses a vessel containing a bath of liquid photo-hardenable material in which the bath surface is maintained at a constant distance from the light source by replenishing the photohardenable material as required when the bottom of the bath necessarily moves downwardly as successive layers are formed to produce the desired three-dimensional object. A workpiece support surface is mounted to move downwardly during the layer forming procedure by a distance coordinated with the amount of liquid discharged from the liquid supply source. The distance moved and amount of liquid added must be precisely coordinated to form the thin layer over the support surface to be exposed to a preselected design of light rays. Otherwise the desired solidified layer of hardened formulation would be inoperative. Through a succession of the disclosed steps, a plurality of exposed transparent film layers are successively stacked in response to the light phenomena recorded for exposing the successive photographic layer. The general concepts used in all stereolithography processes are thus in the public domain.
The patent issued to Fudim describes the use of a transmitting material to fix the surface of a photopolymer liquid to a desired shape, assumably flat, through which photopolymers of desired thickness are solidified. The transmitting material is usually rigid and either coated or inherently nonadherent to the solidified photopolymer. The methods described by Fudim do not address the problems inherent in separating such a transmitting material from a photopolymer formed in intimate contact with the surface of the transmitting material. Whereas the effects of chemical bonding may be reduced significantly by suitable coatings or inherently suitable films, the mechanical bonds along with hydrogen bonds, vacuum forces, and the like are still present and in some cases substantial enough to cause damage or distortion to the photopolymer during removal from the transmitting material surface.
The methods using doctor blades and/or material supply mechanisms shown in Japanese Patent Application Publication Kokai Nos. 61-114817, 61-114818, and 61-116322 require an exact amount of material or photoformable composition to be added in the vessel every time a layer has to be formed. Also, they require the doctor blade or smoothening blade to have a length equal to the width of the vessel in order to properly operate. Because of this, the systems described in these patents have restrictions necessarily confining the photo-sensitive material between the doctor blade and part of the vessel at all times. Thus, it becomes very difficult to form a uniform layer in one continuous pass of the doctor blade without ending up with an excess or shortage of material at the end of the pass. In other words, the doctored layer may be either lacking a part of it at the end of one doctoring operation or it may have an excess of material, which will be very difficult to redistribute in order to achieve the proper thickness and uniformity, due to the confined nature of the arrangement. Also, the doctoring blade has a tendency to create wave motion in the material surrounding the previously exposed layer causing a disturbing effect, particularly on parts of the previously exposed layer which are partially unsupported.
U.S. Pat. No. 3,428,503 discloses a three-dimensional reproduction method comprising the successive masking and etching away of material from a block. The block is coated with successive layers of photo-sensitive resist material used to prepare the block for etching away each layer with a liquid etching fluid disposed in a vat. In effect, patent '503 discloses a method for removing a successive number of contiguous layers of material that produces a mirror image of the Munz three-dimensional object. The resultant cavity thus formed in the block is then used to produce a casting.
An article entitled "Automatic Method for Fabricating a Three-Dimensional Plastic Model with Photo-hardening Polymer" by Hideo Kodama, Rev. Sci. Instrum. 52(11), 1770-1773, November 1981, discloses a method for forming a three-dimensional model by stacking cross-sectional layers formed within a bath of liquid photo-hardenable polymer that is selectively exposed to ultraviolet rays. The computer-controlled layer formation is automatic. Three-dimensional cross-sectional shapes stored in the computer determine the selected shape of the area to be exposed to the ultraviolet rays. The liquid polymer level in the vat is automatically adjusted as the workpiece support surface is moved upwardly or downwardly depending on whether the direction of the ultraviolet light exposure is from the top or bottom of the polymer-containing vessel.
In another Kodama embodiment, a scanning fiber transmitter mounted on an x-y plotter directs the ultraviolet rays to the surface of the resin being exposed in the pattern of the cross-section of the object being formed in the liquid polymer bath. The support surface is intermittently immersed in steps to automatically maintain a constant distance between the light source and top surface of the bath upon solidification of each successive layer. The Kodama system shows the making of complex three-dimensional shapes including the formation of an adequately cohesive layer of structure capable of being partially unsupported by any other layer of structure during formation.